Dating Fossils

Categories of dating

Absolute - determines actual age of specimen in years

Relative dating - tells whether one sample is older or younger than the other

Age of fossils

Given in years before present e.g 45000 BP

Absolute dating methods

Isotopes, potassium-argon dating

Isotopes

All atoms of an element have the same number of protons, called the atomic number. Isotopes are atoms of the same elemnt with different numbers of neutrons.

Nuclear decay

Some isotopes are unstable, may lose protons or neutrons. Rate at which certain isotopes decay can be used to determine age of fossils/artefacts

Potassium-argon dating

Based on decay of radioactive potassium to form calcium and argon. Determining amounts of potassium-40 and argon-40 in a rock sample enables us to calculate the age.

Potassium

One of the most abundant elements in Earth's crust and is therefore found in some rocks. Mixture of 3 different isotopes, all have 19 protons.

Potassium-40

radioactive isotope, decays to form calcium-40 and argon-40, decays at an extremely slow but constant rate

How potassium-argon dating works

Argon is a gas -> assumed that when rock is formed there will be none present in the sample (would have bubbled out during liquid phase of magma). As time goes by, potassium will decay into argon -> increases ratio of argon to potassium, which gives us an approximate age for the fossil within a narrow range

Usefulness of potassium-argon dating

Limited as not all rock types are suitable for this method of dating - can only date rocks older than 200,000 years because half-life of potassium is 1.25 billion years. After 100,000 years, only 0.0053% of potassium would have decayed to argon-40, which is hard to detect.

Needed for potassium-argon dating

Suitable rock of same age as the fossil must be available

Carbon-14 dating

Based on decay of radioactive isotope of carbon-14 to nitrogen-14. Nitrogen-14 exposed to cosmic radiation is converted to carbon-14, produced in the upper atmosphere.`

How carbon dating works

Ratio of one carbon-14 atom to a trillion carbon-12 atoms (more stable). During photosynthesis, one carbon atom in every trillion will be carbon-14, which is incorporated into plant tissues. When an organism consumes that plant, carbon atoms become part of organism's tissues. If organism dies, carbon-14 in organism begins to decay to nitrogen-14 at fixed rate, while carbon-12 remains stable. Ratio of carbon-14 to carbon-12 can be estimated and age can be calculated

Why not ratio of carbon-14 : nitrogen-14?

Hard to look at ratio of carbon-14 to nitrogen-14 as nitrogen is released as a gas

Carbon dating requirements

Half-life of radioactive carbon - 5730 years, requires at least 3g of organic material so rate of radioactive decay can be measured

AMS

Accelerator Mass Spectrometry - used to date samples as small as 100micrograms. Involves breaking sample up into constituent atoms so number of atoms of each isotope of carbon can be counted, used to date cave paintings from tiny samples of pigment

Limitations of carbon dating

After 70,000 years, quantity of carbon-14 is negligable, thus cannot be used to date back more than about 40,000 years. Material to be dated must contain organic compounds. Amount of carbon-14 in atmosphere to carbon-12 varies, therefore dates must be treated with caution.

Relative dating

Determines if an object is younger/older than another = determines relative age. Qualitative, less specific than absolute dating, less expensive and time consuming than absolute dating, works best in sedimentary rocks, which have a layered arrangement of sediment

Stratigraphy

Study of rock layers (strata). Relies on principle of superposition

Principle of superposition

Any fossils or materials found in higher layers will be younger than those lower down. Assumes fossils within a rock layer or near an index fossila are the same age as that layer/index fossil

Limitations of principle of superposition

Distortions of the Earth's crust do occur. sequence of rock layers may be turned upside down. Possible for fossils/artefacts to be buried by animals/early humans after deposition of sediment - means that it can be younger than layers on top

Correlation of rock strata

Involves matching layers of rocks from different areas. Can be done by examining the rock itself, also by studying the fossils it contains. If sequence of sedimentary rocks in different areas are similar -> likely to be same age. Rocks that contain the same index fossils are also considered to be the same age

Index fossils in stratigraphy

Great value in correlation work, because they are widely distributed and were present on Earth for only a limited period of time

Trilobites

Very numerous and widespread index species of Paelaeozoic era

Pollen

Analysis of fossilised pollen grains is also useful so botanists can picture type/amount of vegetation existing at the time and climatic conditions can be worked out. Data can be used to confirm/refute relative dates arrived at by other methods